For many rivers and canals, you apply online to the Canal and River Trust. Dead simple, and you can print out your day ticket on the day.
https://canalrivertrust.org.uk/enjoy...isitor-licence
For rivers not on the list, you would need to do a Google search. For example, if I want to go on the Ouse near Huntingdon, I found I can either apply in advance, or simply go to one of two marinas that sell tickets over the counter. It's about £8.50 for a day.
I do not object to paying the day ticket, although for a small boat it is likely you would get away with it if you didn't pay. Any navigable river in this country is likely to have had work done on it: dredging to mantain the depth, weirs to maintain the water level, flood banks, moorings etc.